The present invention relates to a printer having a printhead which is detachably mounted on a support plate and is held thereon at two rigid support points located on a first side of the printhead, and one elastic support point located on a second side of the printhead, opposite to said first side, wherein each of the support points is formed by a first portion on the side of the printhead and a second portion on the side of the support plate, and one of the first and second portions is a spherical surface engaged in a recess of the other of the first and second portions in a self-centering manner.
The present invention is applicable to a scanning-type printer, e.g. an ink jet printer, wherein the support plate is provided on a carriage so that it may scan a recording medium in a main scanning direction, while the printhead faces the recording medium and prints individual pixels or sets of pixels. In order to obtain a high print quality, it is essential, that the printhead is stably positioned on the support plate with very high accuracy in all six degrees of freedom of the printhead. The accuracy requirements increase with increasing resolution of the printer and may, for example, only allow tolerances of up to ±3 mm.
In an ink jet printer, for example, the printhead typically has a plurality of nozzles that are arranged in a linear array forming an angle of exactly or approximately 90° with respect to the main scanning direction. The angle between the nozzle array and the main scanning direction must be defined with high precision. Frequently, the printer has a plurality of printheads mounted on the same support plate, e.g. printheads for different colors in case of a color printer. Thus, the angular positions of the nozzle arrays must be exactly the same for all printheads. Likewise, the printheads must be arranged with well defined spacings in the main scanning direction and must also be exactly aligned in that direction. Since a small gap is formed between the nozzle surface of the printhead and the surface of the recording medium, the ink droplets expelled from the nozzles must fly a certain distance through the air before they hit the recording medium. Since the carriage is moving, the flight distance of the ink droplets has an influence on the positions of the pixels formed on the recording medium, and, as a consequence, the gap between the printheads and the recording medium must also be defined with high accuracy.
In a conventional printer, the position and posture of the printhead in all six degrees of freedom is defined by six contact surfaces where the printhead engages the support plate, and a spring assembly is used for biasing the printhead against each contact surface. However, if, for any reason, an external force tends to deviate the printhead from the intended position, frictional forces at the contact surfaces may prevent the printhead from returning exactly to its original position. Moreover, if the printhead is detached and is then mounted again on the support plate, cumbersome mounting and adjustment operations are necessary, and these operations, in most cases, cannot be left to the user but require the intervention of a service engineer.
EP-A-0 791 461 discloses an ink jet printer wherein four support points are formed by two pairs of recesses on opposite sides of the printhead, and two spherical surfaces are formed directly on the mounting plate so as to engage the recesses of the first pair, and two spherical surfaces are formed on a detachable clamping plate so as to engage in the recesses of the second pair. The clamping plate can be biased against the mounting plate and is positioned relative thereto by another sphere-and-recess-type support point.